Conventional bicycles with a multispeed transmission of the derailleur type are provided with a gear-shifting mechanism which includes a guide engaging the driving chain and, through selective alignment of that chain guide with a particular sprocket wheel of that transmission, allows a certain speed ratio to be chosen. To change from one speed ratio to another while in motion, the rider operates a handle on the vehicle frame which displaces the chain guide, usually through the intermediary of a Bowden cable, in a direction substantially parallel to the wheel axis. The chain guide carries a spring-loaded tension roller (with or without sprocket teeth) taking up the slack of the chain which varies for the different sprocket wheels. When it becomes necessary to detach the rear axle and its traction wheel from the frame, e.g. for replacement of a tire, the shifting mechanism and the chain remain attached to the frame; the chain, however, must be disengaged from the sprocket assembly and must thereafter be re-engaged therewith, which is a rather laborious procedure.